Method for measuring the thickness of a mail item

ABSTRACT

A method of using reflection of a laser ray (LZ) for measuring the thickness (e) of a flat mail item ( 1 ) consists in nipping the mail item by means of an elastically deformable member ( 3 ) that has a first surface ( 4   a ) in contact with one face of said mail item, and a reflective second surface ( 4   b ) substantially parallel to said first surface, and in directing the laser ray onto said reflective second surface ( 4   b ) for the purpose of measuring the thickness of the mail item.

The invention relates to a method of using reflection of a laser ray formeasuring the thickness of a flat mail item.

The invention is more particularly applicable to measuring thethicknesses of flat mail items that are moved in series, on edge, and athigh speed through a postal sorting installation. Accurate measurementof thickness of such mail items is essential in order to perform thevarious handling operations leading to sorting such as segregation ofmail items on the basis of format, verification of postage amount, orindeed monitoring the filling of the bins or of the joggers in thesorting outlets of the postal sorting installation.

In a postal sorting installation, the thicknesses of mail items shouldbe measured in real time with the constraints that mail items have awide variety of sizes and thus of thicknesses (length in the range 140millimeters (mm) to 400 mm, and thickness in the range 0 to 32 mm). Inaddition, mail items are moved at very high speeds, speeds in common usereaching four meters per second.

Various techniques are already known for measuring the thicknesses offlat mail items in motion in a postal sorting machine.

One known technique is the laser reflection technique which consists insending two laser rays onto respective ones of the two opposite faces ofeach mail item, by means of two laser cells situated facing each otheron either side of the conveyor path along which the mail items areconveyed. The time taken for the laser rays to reflect off the oppositefaces of the mail item serves as a physical magnitude for measuring thethickness of the mail item in differential manner. That measurementtechnique makes it possible to measure only the apparent thickness ofeach mail item. Many mail items are in the form of a wrapper made ofplastic, and, on passing between the two laser cells, the wrapper ofeach such mail item can swell or be deformed so that the apparentthickness measured for said mail item can differ very considerably fromits real thickness, thereby giving rise to errors in the handlingoperations such as format segregation, verification of postage amount,or indeed monitoring of the filling of the bins at the sorting outlets.In addition, that technique is highly dependent on the type and thequality of the outside surface of each mail item off which the laser rayis reflected. It has been observed that measuring by using laserreflection off certain mail items having outside surfaces made of darkplastic gives results that are wildly inaccurate.

Another known technique is based on the use of mechanical feelers. Themail item, as driven by two parallel motor-driven conveyor belts, passesbetween two feelers. The displacement of the feelers relative to theirrest position serves as a physical magnitude for measuring the thicknessof each mail item. The feelers are generally mounted on vertical axesand they are connected mechanically to a sensor for measuring angulardeflection. That type of sensor is unsuitable for measuring thethicknesses of mail items that are moved at high speeds, unless feelersare provided that are made of materials that are ultra-lightweight,highly costly and that withstand mechanical constraints poorly. Inaddition, the friction of the feelers on fragile mail items, etc. mailitems in very fine plastics wrappers, can damage such mail items.

Known measurement techniques are therefore unreliable, costly, andinaccurate, sometimes giving rise to wildly inaccurate results, andgenerating handling errors in a postal sorting installation.

An object of the invention is to remedy the above-presented drawbacks byproposing a method of measuring the real thickness of a flat mail itemthat does not damage the mail item, and that offers a very rapidresponse time, and improved reliability, regardless of the type of themail item.

To this end, the invention provides a method of using reflection of alaser ray for measuring the thickness of a flat mail item moving througha sorting machine, said method being characterized in that it comprisesthe following steps:

-   -   nipping the mail item by means of an elastically deformable        member that is movably mounted and that moves with the movement        of the mail item, said member having a first surface in contact        with one face of said mail item, and a reflective second surface        substantially parallel to said first surface; and    -   directing the laser ray onto said reflective second surface for        the purpose of measuring the thickness of the mail item.

The elastically deformable member may consist of a conveyor belt havinga reflective face or a reflective line on that face which is oppositefrom the face in contact with mail item.

Preferably, in accordance with the invention, said member is anelastically deformable wheel that presents an annular tread strip incontact with one face of the mail item and, on the side opposite fromthe tread strip, a reflective annular strip which is concentric with theannular tread strip and against which the laser ray is directed in orderto perform measurement of the thickness of the mail item in absolutemanner. With such apparatus, in contact with the mail item, the annulartread strip is deformed radially over a distance corresponding to thereal thickness of the mail item, and this deformation distance ismeasured by laser reflection off the reflective annular strip which canhave calibrated pigmentation. The thickness measurement of the inventionis thus independent of the type and of the quality of the outsidesurface of the mail item. This contact-free thickness measurementprinciple is particularly well suited to real time measurement of mailitems that are moved in series, on edge, and at very high speed througha postal sorting machine that is already equipped with passage detectorsfor detecting when mail items go past. Such passage detectors serve tosynchronize the thickness measurements with the passing of the mailitems.

In apparatus of the invention, for using reflection of a laser ray formeasuring the thickness of a flat mail item moving through a sortingmachine, said member is formed by two elastically deformable wheelsdisposed on either side of the mail item. Each elastically deformablewheel presents an annular tread strip and, on the side opposite from thetread strip, a reflective annular strip which is concentric with theannular tread strip. The annular tread strips of the two wheels are incontact with respective ones of the two opposite faces of the mail item,and two laser rays are directed onto respective ones of the reflectiveannular strips of the two elastically deformable wheels, in order toperform measurement in differential manner.

Such measurement apparatus having one or two elastically deformablewheels can further present the following features:

-   -   each laser ray is guided in a tube provided with bends and        provided with mirrors inside it, thereby making it possible to        install the laser cell at a sufficient distance from the zone        off which the laser ray is reflected, for the purpose of        improving the accuracy of the measurement, and outside the        wheel, in particular for facilitating any maintenance        operations; and    -   each elastically deformable wheel is a bladed wheel made of an        elastomer material and in which the reflective annular strip of        the wheel extends from one face of the wheel over a portion of        the width of the wheel that is blade-free.

The method and apparatus of the invention are described in more detailbelow with reference to the drawings, in which:

FIG. 1 very diagrammatically shows an implementation of a method of theinvention with a single elastically deformable wheel;

FIG. 2 very diagrammatically shows another implementation of a method ofthe invention with two elastically deformable wheels;

FIG. 3 is a very diagrammatic view of the path followed by the laser rayin the two elastically deformable wheels shown in FIG. 2;

FIG. 4 is a view from underneath and in perspective, showing apparatusof the invention having two elastically deformable bladed wheels formeasuring the thicknesses of flat mail items;

FIG. 5 is a view from above and in perspective, showing the apparatus ofFIG. 4;

FIG. 6 diagrammatically shows the arrangement of a tube for guiding alaser ray between a laser cell and the reflective annular strip of awheel in the apparatus shown in FIG. 4 or 5; and

FIG. 7 very diagrammatically shows another implementation of the methodof the invention with measurement by laser reflection off a conveyorbelt between two elastically deformable wheels.

In the description, the term “flat item” is used to designate a flatmail item of the letter or “flat” type.

The principle of the invention for using reflection of a laser ray formeasuring the thickness of a flat item that extends in a certainlongitudinal direction consists in nipping the flat item by means of anelastically deformable member having an outside first surface in contactwith one face of the flat item and a reflective second surface that issubstantially parallel to the first surface, and in directing the laserray onto said reflective second surface in a direction that issubstantially perpendicular to the longitudinal direction.

FIG. 1 shows an implementation of the method of the invention making itpossible to measure, with high accuracy, the thickness of a flat item,in particular a flat item in motion. In this example, the thickness ofthe flat item 1, which is disposed on edge, is measured by means of asingle laser reflection. The flat item 1 is nipped between a stationaryplate 2 and an elastically deformable wheel 3. The wheel 3 is a bladedwheel which is mounted to rotate about a stationary axis 4, and it canbe seen in this figure that the wheel 3 has an annular tread strip 4 awhich is in contact with one face of the flat item 1. It can be seen inthis figure that the tread strip 4 a is deformed radially under thepressure exerted by the flat item when it is engaged in the direction Dbetween the fixed plate 2 and the wheel 3, over a distance e thatcorresponds to the thickness of the flat item. It should be understoodthat the tread strip of the wheel comes flush with the reference plate 2without being deformed when no item is inserted between them. The methodof the invention consists in directing a laser ray from a laser cellrepresented by the unit 5 onto the reflective annular strip 4 b that issituated on the inside periphery of the wheel rim on the side oppositefrom the tread strip 4 a. The reflective annular strip is calibrated tobe concentric with (parallel to) the tread strip 4 a so that it issubjected to radial deformation identical to the radial deformation ofthe tread strip when a flat item engages between the reference plate andthe elastically deformable wheel. The laser ray LZ reflected by thereflective annular strip 4 b to the cell 5 makes it possible to obtainan absolute measurement of the distance of said radial deformation, andthus of the real thickness of the flat item. The thickness e of the flatitem 1 can be measured when said flat item is standing still or when itis in motion. In FIG. 1, the flat item 1 is moved in the direction D viaa conveyor belt 6. In the arrangement shown in FIG. 1, the tread strip 4a of the wheel 3 is in contact with the belt 3 so as to nip the flatitem 1 against the reference plane 2.

Naturally, it can be understood that the laser ray indicated by arrow LZis directed radially onto the reflective strip 4 b, i.e. perpendicularlyto the direction D or indeed to the face of the flat item that is incontact with the wheel 3. In particular, the laser ray LZ is directed atthe center of the deformed zone (rectilinear zone) of the tread strip orof the reflective strip.

FIG. 2 shows another implementation of the method of the invention withtwo elastically deformable wheels 3, 3′ mounted to rotate aboutrespective ones of two stationary axes 4, 4′ while being disposed facingeach other so as to nip the flat item 1 between them. In the absence ofa flat item, the two wheels touch each other without any deformation oftheir rims. A laser cell 5, 5′ is associated with each elasticallydeformable wheel and it sends a laser ray LZ, LZ′ onto the reflectivestrip 4 b, 4 b′ of the corresponding wheel in order to perform thicknessmeasurement in differential manner.

In this implementation, the flat item 1 can still be in motion while itsthickness is being measured. In particular, it can be moved between twoconveyor belts represented by 6 and 6′ which are themselves nippedbetween the two wheels 3 and 3′.

For each of the elastically deformable wheels, it is possible to use a“low-pressure” wheel of the type of the wheels described in FrenchPatent No. 2 794 732, which wheels are bladed wheels that are made ofelastically deformable elastomer.

The technique of measuring by laser ray reflection consists in sending alaser ray from a cell onto a reflective surface and in measuring thetime taken by the laser ray to return to the laser cell after it hasbeen reflected off said surface.

A laser cell for implementing the method of the invention can, forexample, be obtained from Baumer insofar as such a laser cell makes itpossible to measure the reflection time of a laser ray accurately,reliably, and with a short response time.

FIG. 3 is a diagrammatic section view showing the structure of twoelastically deformable bladed wheels 3, 3′ which are disposed facingeach other for nipping the flat item 1. In this figure, two conveyorbelts 6, 6′ are shown between which the flat item is nipped. The twoconveyor belts are nipped between the tread strips 4 a, 4 a′ of the twowheels 3, 3′ which can be motor-driven wheels. The axles of the twowheels are fixed to a deck 7 on either side of the conveyor path alongwhich the flat item is conveyed. FIG. 3 shows the blades 4 c, 4 c′ ineach elastically deformable wheel. It also shows that the reflectiveannular strip 4 b, 4 b′ extends from one face of a wheel over a portionof the width of the wheel that is blade-free.

It can also be seen in FIG. 3 that, in the internal space of a wheeldefined by the reflective annular strip, a mirror 8, 8′ angularlypositioned at 45° is disposed to deflect the laser ray LZ, LZ′ radiallyonto the reflective annular strip so that the laser cells 5, 5′ can bedisposed outside the elastically deformable wheels.

FIGS. 4 and 5 show, more precisely, apparatus for measuring, in realtime, the thicknesses of flat mail items moved in series and on edge bya belt conveyor 6, 6′ in a postal sorting installation. In thisapparatus, two elastically deformable bladed wheels 3, 3′ are used, asare two laser cells 5, 5′ which are disposed outside the wheels 3, 3′.Each laser cell sends a laser ray which is guided in a tube 9, 9′provided with bends. Each tube has two right angles (more visible inFIG. 6) at which respective mirrors 8A, 8B are disposed that areangularly positioned at 45° in order to guide the laser ray from thecell to the reflective annular strip of a wheel and vice versa. The tubeprovided with bends and with the mirrors makes it possible to increasethe path length of the laser ray so as to take account of the operatingcharacteristics of the laser cell, so as to protect the laser cell fromany dust flowing in the environment of the wheels, and so as to disposethe laser cell outside the wheels in order to facilitate maintenanceoperations.

From FIGS. 4 to 6, it can be understood that the path of a laser raydelivered by a laser cell is deflected through a right angle at a firstmirror, and is then deflected through a right angle again at a secondmirror before it is reflected off the reflective annular strip of awheel, whereupon it follows the reverse path towards the cell in orderto be retrieved and processed therein. The path is followed through atube in the form of two L-shapes leading one into the other andoccupying two mutually perpendicular planes. Such a tube can be obtainedeasily by molding a plastics material.

As indicated above, use is made of passage detectors, generally presentalong the path along which the mail items are conveyed through thesorting installation, in order to synchronize the thickness measurementstaken on each mail item.

The elastically deformable wheels 3, 3′ can be made of an elastomermaterial giving them good strength over time.

The reflective annular strip of each wheel is constituted by a surfacemade of elastomer whose pigmentation intrinsically presents reflectioncharacteristics that are sufficient, but it is also possible to coversaid elastomer surface with a paint having a higher reflective capacity.

FIG. 7 shows another implementation of a method of the invention, inwhich implementation the measurement by reflection of a laser ray isachieved by sending a laser ray LZ″ by means of a laser cell 5″ (LZ″ and5″ having characteristics similar respectively to those of LZ, LZ′ andto those of 5, 5′) onto a conveyor belt 10. The mail item 1 is moved inthe direction D between two conveyor belts 10, 11 past two elasticallydeformable wheels 12 that have characteristics presented in FrenchPatent No. 2 794 732 and being disposed on the same side of the conveyorpath, in contact with the conveyor belt 10, and spaced apart by adistance substantially identical to the maximum length of a mail item inthe direction D. The tread strips of the wheels 12 are not deformed whenno mail item goes past them. Between the two wheels 12, on the otherside of the conveyor belts 10, 11 from the wheels, a reference pulley 13is arranged which is exactly circular, which turns about its stationarycenter and against which the conveyor belt 11 bears, as does theconveyor belt 10 when no mail item 1 is passing between the two wheels12. When a mail item 1 is situated between the two wheels 12, it isnipped by the two conveyor belts 10, 11 because of the pressure exertedby the wheels 12 on the conveyor belts 10, 11, because of the presenceof the reference pulley 13 which serves as a bearing point, and becauseof the elastic deformation of the conveyor belt 10 further from thereference pulley 13. When the mail item 1 is compressed between the twoconveyor belts 10, 11, the distance between the two belts is equal tothe thickness e of the mail item 1, which thickness also corresponds tothe displacement of the conveyor belt 10 perpendicularly to thedirection D facing the reference pulley 13. The displacement of theconveyor belt 10 is measured by sending a laser ray LZ″ perpendicularlyto the displacement by means of a laser cell 5″ onto that face 14 of theconveyor belt 10 which is not in contact with the mail item 1. The faceonto which the laser ray LZ″ is sent is made of a reflective material oris covered entirely or in part (e.g. over a strip) with a reflectivematerial.

The conveyor belt 11 can be pressed against a reference plate (notshown) or replaced with a reference plate on which the mail item slides.

The implementation shown in FIG. 7 is very easy and inexpensive to putin place in a postal sorting machine.

1. A method of using reflection of a laser ray (LZ, LZ′, LZ″) formeasuring the thickness (e) of a flat mail item (1) moving through asorting machine, said method being characterized in that it comprisesthe following steps: nipping the mail item by means of an elasticallydeformable member (3; 3′, 10) that is movably mounted and that moveswith the movement of the mail item, said member having a first surface(4 a, 4 a′) in contact with one face of said mail item, and a reflectivesecond surface (4 b; 4 b′; 14) substantially parallel to said firstsurface; and directing the laser ray onto said reflective second surface(4 b, 4 b′, 14) for the purpose of measuring the thickness of the mailitem.
 2. The method of claim 1, in which the mail item (1) extends in acertain longitudinal direction (D) and in which the laser ray isdirected onto said reflective second surface in a direction that issubstantially perpendicular to said longitudinal direction (D). 3.Apparatus for using reflection of a laser ray for measuring thethickness (e) of a flat mail item (1) moving through a sorting machine,said apparatus being characterized in that it comprises an elasticallydeformable member (3; 3′, 10) that is movably mounted and that moveswith the movement of the mail item (1), said member having a firstsurface (4 a, 4 a′) in contact with one face of said mail item, and areflective second surface (4 b; 4 b′; 14) substantially parallel to saidfirst surface; and in that it further comprises means for directing thelaser ray onto said reflective second surface (4 b, 4 b′, 14) for thepurpose of measuring the thickness of the mail item.
 4. The apparatus ofclaim 3, in which said member (3; 3′) is an elastically deformable wheelthat presents an annular tread strip (4 a; 4 a′) in contact with oneface of the mail item and, on the side opposite from the tread strip, areflective annular strip (4 b, 4 b′) which is concentric with theannular tread strip and against which the laser ray is directed.
 5. Theapparatus of claim 3, in which said member (3, 3′) is formed by twoelastically deformable wheels disposed on either side of the mail item,each elastically deformable wheel presenting an annular tread strip (4a, 4 a′) and, on the side opposite from the tread strip, a reflectiveannular strip (4 b, 4 b′) which is concentric with the annular treadstrip, the annular tread strips (4 a; 4 a′) of the two wheels being incontact with respective ones of the two opposite faces of the mail item,and in which two laser rays are directed onto respective ones of thereflective annular strips of the two elastically deformable wheels. 6.Apparatus according to claim 4, in which each laser ray is guided in atube (9, 9′) provided with bends and provided with mirrors (8A; 8B)inside it.
 7. Apparatus according to claim 4, in which each elasticallydeformable wheel (3, 3′) is a bladed wheel made of an elastomer materialand in which the reflective annular strip of the wheel extends from oneface of the wheel over a portion of the width of the wheel that isblade-free.
 8. The apparatus of claim 3, in which said member is formedby a conveyor belt (10), said conveyor belt (10) having one face incontact with the mail item (1), and, on the side opposite from the facein contact with the mail item, a face (14) with a reflective surfaceonto which the laser ray (LZ″) is directed.
 9. A postal sorting machinecomprising a belt conveyor (6,6′) suitable for moving flat mail items(1) on edge, and apparatus according to claim 4, disposed such that thetread strip of each elastically deformable wheel (3; 3′) is in contactwith a conveyor belt.